1. Field of the Invention
The present invention relates generally to straddle tools for use in wellbores for stimulation or fracturing of packer isolated annulus intervals and more particularly to straddle tools having valves that are actuated to cause dumping into the well below the straddle tool fluids from a conveyance and injection tubing string, from the straddle tool and from the annulus interval being treated. More particularly, the present invention concerns valves are operated by flow and controlled by indexing to accomplish selected valve positioning to provide for interval treatment and to provide for dumping of treatment fluid from a tubing string, from the straddle tool and from the annulus intervals upon completion of well interval treatment and to prevent flow responsive valve movement under certain conditions.
2. Description of the Prior Art
After a wellbore is drilled, various completion operations are performed to enable production of well fluids. Examples of such completion operations include the installation of casing, production tubing, and various packers to define zones in the wellbore. Also, a perforating string is lowered into the wellbore and fired to create perforations in the surrounding casing and to extend perforations into the surrounding formation.
To further enhance the productivity of a formation, fracturing may be performed. Typically, fracturing fluid is pumped into the wellbore to fracture the formation so that fluid flow conductivity in the formation is improved to provide enhanced fluid flow into the wellbore. Enhancement of well production is also achieved by chemical treatment, such as acidizing, through the use of similar well treatment straddle packer tools.
A typical fracturing string includes an assembly carried by tubing, such as coiled tubing or jointed tubing, with the assembly including a straddle packer tool having sealing elements to define a sealed annulus interval between the assembly and the well casing into which fracturing fluids can be pumped. The well casing of sealed or isolated annulus interval is perforated for communication with the surrounding formation. The fracturing fluid is pumped down the tubing and through one or more ports of the straddle packer tool into the sealed annulus interval.
After the fracturing operation has been completed, clean-up of the wellbore and coiled tubing is performed by pumping fluids down an annulus region between the coiled tubing and casing. The annulus fluids push debris (including fracturing proppants) and slurry present in the interval adjacent the fractured formation and in the coiled tubing back out to the well surface. This clean-up operation is time consuming and is expensive in terms of labor and the time that a wellbore remains inoperable. By not having to dispose of slurry, returns to surface are avoided along with their complicated handling issues. More importantly, when pumping down the annulus between coiled tubing and the wellbore, the zones above the treatment zone can be damaged by this clean-out operation. Further, under-pressured zones above the straddled zone can absorb large quantities of fluids. Such losses may require large volumes of additional fluid to be kept at surface for the sole purpose of clean-up. An improved method and apparatus is thus needed for performing clean-up after a fracturing operation has been completed.
Prior well treatment tool designs involved the use of a well treatment and slurry removal tool that could only open or close; and with no intermediate positions between the open and closed positions. This tool used a pressure drop across an orifice to load a compression spring to close the valve. Once closed, differential pressure between tubing pressure and wellbore annulus below the treated zone keeps the valve closed. Reduction of that differential pressure across the valve allows the tool to open. However, this severely limits the application and usage of this tool in demanding well conditions. For instance, in order to use this device in wells with low bottom hole pressures, a large spring is used. However, a high flow rate is needed to close the tool with this large spring. This proved to be a problem due to many reasons. Also, this design does not allow operation in wells with bottom-hole pressures below a certain value and fracture gradients below a certain value.